In certain image forming apparatuses, a constant current control is performed to maintain a transfer current flowing through a transfer nip at a predetermined value. In this type of image forming apparatus, the use of a coarse sheet with relatively large surface irregularities as a recording media tends to result in images with an uneven image density, making the image density lower in regions corresponding to recesses in a surface of a recording sheet than in regions corresponding to projections on the surface of the recording sheet.
To minimize unevenness in image density, the control target value for the transfer current can be increased in accordance with an increase in an image area ratio of a toner image entering the transfer nip. The transfer current is thus controlled for the following reason. That is, if left uncontrolled, a relatively large transfer current flows to the toner image on the intermediate transfer member in the transfer nip, discharge occurs in an air gap between a recess in the surface of the recording sheet and a portion of the toner image facing the recess, and toner in the portion of the toner image is oppositely charged. As a result, most of the toner in that portion of the toner image fails to be electrostatically transferred to the recess in the surface of the recording sheet, and thus the above-described unevenness in image density becomes prominent. In the transfer nip, therefore, it is desired not to apply an excessive amount of transfer current to the toner image.
With constant current control of the transfer current, however, a change in the image area ratio of the toner image entering the transfer nip results in a substantial change in the amount of current flowing into the toner image. Specifically, under a condition of controlling to maintain a constant output value of the transfer current from a power supply, the lower the image area ratio of the toner image entering the transfer nip, the larger the transfer current flowing into the toner image. As a result, the above-described discharge tends to occur. Conversely, the higher the image area ratio, the smaller the transfer current flowing into the toner image. As a result, a deficiency in image density tends to occur in the recesses in the surface of the recording sheet. To counteract this effect, as described above a larger transfer current is deliberately output in accordance with the increase in the image area ratio, to thereby apply an appropriate transfer current to the toner image regardless of the image area ratio.
In addition, the size of the transfer current flowing to the toner image in the transfer nip is affected by the electrical resistance of the recording sheet as well as by the image area ratio. Specifically, in the use of a recording sheet substantially reduced in electrical resistance owing to moisture absorption in a relatively highly humid environment, the transfer current more easily flows into an area of direct contact between a non-image area on the intermediate transfer member (i.e., an area in which no image is present) and the recording sheet than in the use of a recording sheet which has not absorbed a relatively large amount of moisture. As a result, the transfer current flowing to the toner image is reduced.
Accordingly, the recording sheet having absorbed a relatively large amount of moisture in a relatively highly humid environment may be subjected to a preliminary sheet feeding operation that is designed to remove moisture from the recording sheet, thereby enabling an appropriate transfer current to be applied to the toner image entering the transfer nip regardless of the initial moisture absorption state of the recording sheet. As a result, unevenness in image density is minimized.
In the above-described configuration, however, the preliminary sheet feeding operation is performed in addition to the actual printing operation. Therefore, the print time is extended. Further, if a preliminary heating device specifically for preliminary heating of the recording sheet is provided on a sheet feed path extending from a sheet feeding cassette to the transfer nip so as to prevent the extension of the print time, the initial cost and running costs of the image forming apparatus are increased.